Tank venting

Storage. Purified and dry aHyl chloride can be safely stored in carbon steel vessels. Use of lined vessels is recommended if slight discoloration or trace presence of metals is undesirable for its intended use. In any event, the presence of air should be avoided for safety (flammabHity) reasons through the use of an inert gas pad. Tank vents should be treated, eg, by incineration, prior to venting to the atmosphere. Some commercial producers intentionaHy add about 0.1% propylene oxide as a stabilizer to prevent discoloration however, this is usuaHy unnecessary if product purity is sufficiently high. 

Applications The common Heliflow apphcations are tank-vent condensers, sample coolers, pump-seal coolers, and steam-jet vacuum condensers. Instant water heaters, glycoLwater seivdces, and cryogenic vaporizers use the spiral tube s ability to reduce thermally induced stresses caused in these apphcations. 

Deflagration Arresters The two types of deflagration arrester normally considered are the end-of-line arrester (Figs. 26-23 and 26-24) and the tank vent deflagration arrester Neither type of arrester is designed to stop detonations. If mounted sufficiently far from the atmospheric outlet of a piping system, which constitutes the unpro-tec tea side of the arrester, the flame can accelerate sufficiently to cause these arresters to fail. Failure can occur at high flame speeds even without a run-up to detonation. 

Deflagration Arrester Testing For end-of-line and tank vent flame arresters, approval agencies may require manufacturers to provide users with data for flow capacity at operating pressures, proof of success during an endurance burn or continuous flame test, evidence of flashback test results (for end-of-hne arresters) or explosion test results (for in-line or tank vent arrester applications), hydrauhc pressure test results, and results of a corrosion test. 

Safety Considerations Design and location of storage tanks, vents, piping, and connections are specified by state fire marshals, underwriters codes, and local ordinances. In NFPA 30, Flammable and Combustible Liquids Code, 1993 (published by the National Fire Protection Association, Quincy, Mass.), liquid petroleum fuels are classified as follows for safety in handhng ... 

Elame Arrester A flame arrester is a device permeable to gas flow but impermeable to any flame. It quenches the flame and cools the products sufficiently to prevent reignition at arrester outlet. Arresters are used to prevent a flame propagating into the system from outside (such as via a tank vent) or one part of the system to another (such as through connected piping). 

While process and equipment modification are generally the preferred alternatives for reducing emissions from a plant, some form of control is necessary before emissions are discharged into the environment. Technologies discussed in this section are applicable in preventing emissions from point sources such as process or tank vents. These technologies fall into two main categories ... 

Canister-type adsorbers differ from fixed-bed units in that they are normally limited to the removal of low-volume, intermittent gas streams, such as storage-tank vent gases. Process economics usually dictate whether regenera-ble or throw-away canisters are appropriate. Each canister unit consists of a vessel, adsorbent, fan (not always necessary), inlet connection and distributor, and an outlet connection for the purified gas. The disadvantage in using canisters is that poor operating efficiencies result if the adsorber becomes saturated. Because the adsorber will probably be disposed of, there is a temptation to operate it until the adsorber is saturated. Unlike fixed-bed units, the concentration of the outlet gases is not usually monitored. ... 

Condensation is normally used for the recovery of organic compounds from process or tank vent gases or from releases during loading. Condensation is used to recover valuable compounds prior to incineration, or to reduce the organic load entering other control systems, such as adsorbers or absorbers. Adsorption and absorption processes benefit from low condenser outlet temperatures. 

While a plant was on line, an operator noticed a slip-plate on a tank vent. The slip-plate had been fitted to isolate the tank from the blowdown system while the tank was under maintenance. When the maintenance was complete, the slip-plate was overlooked. Fortunately, the tank, an old one, was stronger than it needed to be for the duty, or it would have burst. 

Figure 2-6. Liquid ethylene tank venting arrangements.

A fire in a bulk storage facility at Coode Island, Melbourne, Australia, in August 1991 caused extensive damage and many complaints about the pollution caused by the smoke plume, but no injuries. The tank vents were connected together and piped to a carbon bed vapor recovery system. There were no flame arrestors in the pipework. Whatever the cause of the initial fire or explosion, the vent collection system provided a means of spreading the fire from one tank to another. 

If the tank vent is not large enough to pass the liquid inlet rate, then the tank should be fitted with a hinged manhole cover or similar overflow device. Proprietary devices are available. 

Lactory Mntnal Research (formerly Lactory Mntnal Research Corporation) pnblished a test procedure for flame arresters for storage tank vent pipes in 1990 (LMRC 1990). The Lactory Mntnal Research test protocol... 

The American Society for Testing and Materials (ASTM) first published a specification for tank vent flame arresters, FI273, in 1991 (ASTM 1991). This was reaffirmed in 1996. This specification provides the minimum requirements for design, constrncdon, performance, and tesdng of tank vent flame arresters (deflagradon flame arresters). 

ASTM F-1273-91. 1991. Standard Speeifieation for Tank Vent Flame Arresters. American Society for Testing and Materials, West Conshohocken, PA. 

EMRC (Eactory Mutual Research Corporation) Class 6061. 1990. FMRC Test Procedure for Flame Arresters for Storage Tank Vent Pipes. Eactory Mutual Research Corporation, Norwood, MA. 

Appendix B to Part 154 is a standard specification for tank vent (end-of-line) deflagration flame arresters. It does contain test procedures for these types of flame arresters. An end-of-line flame arrester mnst be able to arrest three atmospheric deflagrations as well as to withstand two honrs of continnons bnrning withont flashback. 

The UL 525 test requirements for tank vent deflagration flame arresters are as follows ... 

The second standard is a procedure for examination and testing of detonation flame arresters (FMRC 1999). Testing to ascertain ability to stop deflagrations and detonations is conducted in accordance with the USCG requirements as outlined in 33 CFR, Appendix A to Part 154. However, the endurance burning test is conducted in accordance with the FMRC test procedure for flame arresters for storage tank vent pipes. 

ASTM published a standard specification F 1273 for tank vent flame arresters (end-of-line deflagration flame arresters) in 1991 (reapproved in... 

End-of-Line Flame Arrester A deflagration flame arrester that is installed on atmospheric pressnre tank vent nozzles, having only one pipe connection. It is also called a tank vent deflagration flame arrester. 

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